As external memory devices used in a computer system, an SSD (Solid State Drive), on which a nonvolatile semiconductor memory such as a NAND-type flash memory is mounted, attracts attention. A flash memory has advantages such as high speed and lightweight compared with a magnetic disk device. An SSD includes therein a plurality of flash memory chips, a controller that performs read/write control of each flash memory chip according to a request from a host apparatus, a buffer memory for performing data transfer between each flash memory chip and the host apparatus, a power circuit, a connection interface to the host apparatus, and the like.